In a statement on October 9, Miklos Haraszti expressed disappointment regarding last month's Supreme Court ruling to uphold a death sentence handed to 23-year-old Pavel Selyun. Selyun was convicted of killing his wife and her lover in August 2012.
Two other death sentences were handed down in Belarusian courts this year for murder charges.
Five executions were held in the period 2010-12.
Haraszti said it was "unacceptable" that "nontransparent and politically guided courts hand down death-penalty sentences at the end of a procedure without guarantees of a fair trial."
He also described the lack of transparency and statistics on executions in Belarus as "deplorable."
Belarus is the only country in Europe that retains the death penalty.